In the chemical industries there is a great need for pipe capable of handling highly corrosive and/or abrasive liquids, slurries and gases, particularly at elevated temperatures. Pipe corrosion problems are also chronic in such fields as food handling and processing, electroplating, metal pickling and petroleum refining. The pipe produced by the process of this invention has utility in all of these fields.
Pipe commonly used today for handling corrosive or abrasive materials comprises a liner formed of a thermoplastic material which is substantially chemically inert to the material to be conducted through the pipe, surrounded by one or more layers of a reinforcing material, such as steel, and glass fibers impregnated with a thermosetting resin.
A particular problem encountered in the manufacture of such pipe is the bonding of the liner of thermoplastic material to the reinforcing layers which is particularly essential for vacuum service. More particularly, fluorinated polymers and polyolefins which are highly resistant to chemical attack, do not lend themselves to bonding to other materials, such as a supporting resin impregnated fiber sleeve, by means of ordinary adhesives. For this reason efforts have been directed to obtaining a physical or mechanical bond between the exterior surface of the thermoplastic liner and a reinforcing sleeve.
According to U.S. Pat. No. 4,104,095 granted Aug. 1, 1978 to William D. Shaw, such a mechanical bond between a thermoplastic liner and a surrounding sleeve formed of a woven glass fabric can be obtained by applying to a cylindrical liner of thermoplastic material a woven glass fiber tape, and externally applying heat to the outer surface of the liner through the sleeve. The heat causes the outer surface of the liner to melt and, by reason of the greater coefficient of thermal expansion of the liner, as compared to the glass fiber tape, the liner expands radially to a greater degree than the tape, whereby the molten exterior surface of the liner enters the interstices of the glass fabric. Upon cooling, there is obtained a mechanical bond between the liner and glass fabric tape. A further exterior reinforcing layer, as for example a layer of a fiber-reinforced thermosetting resin, can then be bonded directly to the first glass fiber tape. In this way the problem of direct bonding of a reinforcing layer to a thermoplastic liner by means of adhesives is avoided.
In the process of U.S. Pat. No. 4,104,095 the mechanical bonding of the liner to the reinforcing glass fiber tape is obtained without introduction of a gas under pressure to the liner during the heating step. Rather, expansion of the molten surface of the thermoplastic liner into the interstices of the reinforcing glass fiber tape is due entirely to the difference in the coefficients of thermal expansion of the liner and reinforcing tape.
The process of U.S. Pat. No. 4,104,095 has been found wanting in a number of respects. Composite pipe having a diameter greater than 6" cannot be made without some deformation or collapse of the liner. The process cannot be used successfully where the reinforcing layer comprises a knitted fabric, for such fabric, by reason of its structure readily expands radially along with the liner. In addition, composite pipe made by the method of U.S. Pat. No. 4,104,095 generally evidences a great deal of bow or warpage.
Another process which has been suggested for the manufacture of seamless laminated pipe comprises inserting in a cylindrical glass fabric sleeve a liner of a thermoplastic polymer, the external surface of which liner has applied thereto an adhesive which is a polymer similar to that of the liner, but has a higher melt index number than that of the liner per se. The resulting assembly is placed in a hollow cylinder and heated to cause the adhesive to soften. Gas pressure is applied within the liner to cause the liner to expand radially so that the adhesive enters the interstices of the glass fabric sleeve. Upon cooling the entire assembly while so expanded, there is obtained a unitary structure, the liner being mechanically bonded to the glass fabric sleeve in essentially the same manner as described in U.S. Pat. No. 4,104,095.
The last-described process also has a number of shortcomings. The expansion of the tubing by means of gas pressure is sufficient to produce residual stresses in the liner which may be relieved in service, resulting in separation of the liner from the surrounding reinforcing glass fabric sleeve. By reason thereof, the pipe loses its vacuum service capabilities. The heating and cooling steps of the process are carried out in a cylindrical chamber which is expensive to fabricate and, for practical handling reasons, must have a limited length, thereby restricting the length of pipe that can be made. Also, the process requires precise temperature control and involves the use of extended heating and cooling cycles.